Low power phototransistor-based welding helmet providing reduced sensitivity to low intensity light and sharp phototransistor response to high intensity light

ABSTRACT

An auto darkening eye protection device including a shutter assembly, a light sensing circuit, a control circuit and a power supply source. The shutter assembly is adjustable to a plurality of shade levels. The phototransistor of the light sensing circuit senses light from a welding arc and provides an output of the light sensing circuit indicative of the shade level at which the shutter assembly should be operated. The phototransistor is configured for surface mount and has an external base connection connected to the base of the phototransistor. The control circuit is configured to receive the output from the light sensing circuit and provide a drive signal to the shutter assembly responsive to said output, drives the shutter assembly to one of said plurality of shade levels. The present invention provides reduced power consumption, improved attenuation of low intensity light signals, a sharp rise time from the phototransistor in response to high intensity light, and allows implementation into a smaller sleeker eye protection device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a continuation of U.S. Ser. No.10/827,014, (U.S. Pat. No. 6,855,922), filed on Apr. 19, 2004, which isa divisional patent application of U.S. Ser. No. 09/659,100, (U.S. Pat.No. 6,815,652) filed on Sep. 11, 2000, the entire content of bothapplications are hereby expressly incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of auto-darkening eyeprotection devices, such as welding helmets having a shutter (or lens)assembly that automatically darkens upon the detection of a welding arc.A photosensitive device such as a photodiode or a phototransistor may beused to sense the intensity of light incident on the area of the shutterassembly so as to provide an indication to the circuitry controlling theshutter assembly that the shutter assembly needs to be driven to eithera dark state or a clear state. If a welding arc is present, the weldinghelmet protects the eyes of the welder from any danger caused by theintensity of the welding arc by driving the shutter assembly to a darkstate, thereby decreasing the amount of energy passing through the lensto the welder's eyes. U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243,Re. 32,521, 5,248,880, 5,252,817, 5,347,383, 5,533,206, 5,751,258,5,959,705, 6,067,129, and 6,070,264 each disclose various shutterassemblies and liquid crystal driver electronics that can be used inconjunction with the present invention. The disclosures of theseabove-mentioned patents are hereby incorporated in their entireties byreference.

Commonly-owned U.S. Pat. No. 5,347,383 discloses a driving circuit for aliquid crystal shutter. The sensor circuitry of this invention utilizesa photodiode to detect the occurrence of welding. This sensor circuitryalso utilizes a comparator to compare the sensed light signal with athreshold value to determine whether the shutter assembly should bedriven to a dark or clear state. Additionally, the '383 patent disclosesthe use of a 9 V supply.

While the invention disclosed in this patent functioned for its intendedpurpose, a need was felt for an improvement in the power consumption bythe sensor circuit. As incident light increases on a photodiode, thevoltage across the photodiode will begin to saturate. To prevent thephotodiode from saturating, a steadily increasing load must be put onthe photodiode which leads to excessive power consumption.

To alleviate the excessive power consumption inherent in aphotodiode-based sensor circuit, a phototransistor has been utilized asa weld sensor. The use of a phototransistor allows the use of feedbackto bias the phototransistor so that less current is needed to keep thephototransistor in its operational mode. Commonly-owned U.S. Pat. Nos.5,252,817, 5,248,880, 5,751,258, and 6,070,264 are illustrative ofsensor circuits using phototransistors as weld sensors. Each of thesepatents discloses a sensor circuit wherein the output of thephototransistor is fed into a comparator. The comparator compares thephototransistor output with a threshold level. If the phototransistoroutput exceeds the threshold level, the drive circuitry is activated todarken the shutter assembly. If the phototransistor output does notexceed the threshold level, the drive circuitry operates the shutterassembly in a clear state. While the circuits disclosed in these patentsutilize feedback to bias the phototransistor and avoid the excessivedrawing of current, heavy loads were still needed. The circuitsimplementing such designs used voltage supplies ranging from 5.6 V to 9V. Therefore, a need was still felt for a sensor circuit having improvedpower consumption characteristics.

Moreover, the phototransistors used in the prior art designs were metalcan phototransistors. Metal can phototransistors are relatively big andbulky. Their size, height and relative difficulty in mounting serves asa limiting factor in the ability of designers to reduce the size of theunits in which the sensor circuit is implemented. Thus, a need was feltto use a smaller and more compact phototransistor that is more easilymountable to a circuit board to produce a smaller, sleeker unit whilestill having the ability to maintain a constant signal level withoutexcessive loading or the drawing of excessive current.

Additionally, the sensor circuits of the prior art produced an outputvoltage from the phototransistor in response to incident light intensityas seen in FIG. 3 of the '880 and '817 patents. As can be seen, lowlight intensities produce a steep rise in output of the phototransistor.Because of the power drain caused by the response of the phototransistorto low intensity incident light, it is desirable that thephototransistor be configured to minimize the phototransistor outputsignal when the sensor circuit is exposed to low intensity incidentlight. Thus, a need existed for a sensor circuit that provided greaterattenuation in the response of the phototransistor to low intensityincident light.

While it is desirable to minimize the phototransistor output when thesensor circuit is exposed to low intensity incident light, thephototransistor still must be able to quickly increase its output inresponse to a transition from low intensity light to high intensitylight, such as the light provided by a welding arc. Thus, an everpresent need exists within the art to sharpen the rise provided by thephototransistor in response to sharp increases in light intensity.

Also, the sensor circuitry of the prior art used a comparator tocorrelate the sensed light signal with the desired shade level. Thecomparator compared the output of the phototransistor with a thresholdvoltage signal to determine whether the shutter assembly should bedriven to a dark state or a clear state. This design required additionalcircuitry to set the threshold voltage level. This additional circuitrynot only complicated circuit design, but also increased the drain on thepower supply. Thus, a need was felt to simplify the sensor circuitry toprovide a more power-efficient way of correlating the phototransistoroutput to the proper shade setting of the shutter assembly.

BRIEF SUMMARY OF THE INVENTION

In order to solve these and other problems in the prior art, theinventor herein has succeeded in designing and developing an improvedwelding detection circuit utilizing a novel phototransistor-based sensorcircuit. This sensor circuit comprises a phototransistor biased via afeedback circuit and having an output connected to an amplifier. Thesensor circuit can be connected to a power supply and a control circuitto drive a shutter assembly to either a dark state or a clear statedepending upon the intensity of incident light.

One feature of the present invention is the use of a resistor coupledbetween the base and emitter of the phototransistor. This resistor helpsreduce the current produced by the sensor during low ambient lightconditions, thereby attenuating the phototransistor output in responseto low intensity light signals, and helps produce a sharply risingvoltage from the phototransistor in response to high intensity lightsignals. Preferably, the feedback circuit also includes a secondresistor coupled between the emitter of a feedback transistor and groundto further attenuate phototransistor output in the presence of lowintensity ambient light.

Another feature of the present invention is its use of a planarphototransistor. Because of the planar phototransistor's small size, ascompared to the metal can phototransistors used in the prior art, andbecause of the planar transistor's ability to maintain a constant signallevel without excessive loading or the drawing of excessive current, theuse of a planar phototransistor not only performs as well as metal canphototransistors, but also allows a reduction in the size of the unit inwhich the circuit is implemented. Preferably, the planar phototransistoris configured for a surface mount to further simplify construction ofthe circuit.

Another feature of the present invention is its use of a closed loopnoninverting amplifier to provide a gain for the phototransistor output.The gain of the amplifier is preferably set so that a sufficient outputvoltage will be generated to activate the shutter assembly when thephototransistor produces an output indicative of the presence of awelding arc. Preferably, a capacitor is coupled between thephototransistor output and noninverting input of the amplifier to blockthe DC portion of the phototransistor output.

Another feature of the present invention is its use of the energy savedby an improved and efficient circuit design to recharge a rechargeablebattery. By recharging the battery, the present invention extends thebattery life of the invention's power supply.

Another feature of the present invention is its use of a solar cell toreduce the circuit's power consumption. By using a solar cell to powervarious components of the circuit, the present invention prevent thosecomponents from acting as a drain on the power supply when the inventionis left unexposed to light. Often, while not in use, a welding helmetwill be left in a dark room or left face down on a table. When in theseconditions, it is undesirable for the circuit to operate as a drain onthe power supply. When, the welding helmet is in use, it will be eitheroutdoors, in a lighted room, or in a dark environment with the presenceof welding arc. In such conditions, it is desirable to use the lightincident on the welding helmet to power the circuitry therewithin.

The present invention uses the solar cell to power the phototransistorand the amplifier that is coupled to the output of the phototransistor,thus preventing those two components from draining the power supply whenthe welding helmet is left unexposed to light.

The present invention also uses the solar cell to power an activationcircuit, the activation circuit functioning to activate a signalgenerator. The signal generator, once activated, generates the voltagelevel and frequency signal to be used to drive the shutter assembly to adark state. The generation of this signal acts as a drain on the powersupply. By using the solar cell to power the activation circuit, thepresent invention improves the circuit's power consumption by triggeringthe signal generator when light is incident on the welding helmet.

Yet another feature of the present invention is its use of a selectorcircuit for selecting the drive signal that will be delivered to theshutter assembly. If the sensor circuit indicates to the selectorcircuit that a welding arc is present, the selector circuit will cause adark state drive signal to be delivered to the shutter assembly. If thesensor circuit indicates to the selector circuit that no welding arc ispresent, the selector circuit will cause a “clear state” drive signal tobe delivered to the shutter assembly. The selector circuit uses atransistor as a switch to control the selection of the drive signal. AnRC circuit is part of the selector circuit. The RC circuit utilizes itsRC time constant to delay the transition of the “dark state” drivesignal to the “clear state” drive signal, thus preventing the shutterassembly from switching to a clear state during brief “off” periods inthe weld pulsations that exist with various weld types.

While the principal advantages and features employed are explainedabove, a fuller understanding of the invention may be attained byreferring to the drawings and description of the preferred embodimentwhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the circuit of the present invention;

FIG. 2 is a schematic diagram of the circuit of the present invention;

FIG. 3 a is a schematic diagram of an equivalent circuit for the sensorcircuit when both the phototransistor and feedback transistor are “off.”

FIG. 3 b is a schematic diagram of an equivalent circuit for the sensorcircuit when the phototransistor is “on” and the feedback transistor is“off.”

FIG. 3 c is a schematic diagram of an equivalent circuit for the sensorcircuit when both the phototransistor and the feedback transistor are“on.”

FIG. 4 is a graph depicting the voltage from the phototransistor as afunction of incident light intensity.

FIG. 5 is a perspective view of a surface mount phototransistor utilizedin the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A block diagram of the circuit of the present invention is depicted inFIG. 1. As can be seen, a power supply 250 is connected via power lines251, 252, 253, 254, and 255 to the sensor circuit 200, activationcircuit 326, selector circuit 316, signal generator 325, and deliverycircuit 315. The power supply 250 furnishes the circuit with the powernecessary for operation. Activation circuit 326, selector circuit 316,signal generator 325, and delivery circuit 315 function together tocontrol the shutter assembly 400 depending upon the signals receivedfrom power supply 250 and sensor circuit 200.

Activation circuit 326 receives power from the power supply 250 andsends an activation signal to the signal generator 325. Upon activationby the activation circuit, the signal generator 325 generates afrequency signal 102 and a voltage signal 104 and sends the two signals102 and 104 to the delivery circuit 315. The delivery circuit 315 usesthe frequency signal 102 and the voltage signal 104 to assemble a “darkstate” drive signal for the shutter assembly 400.

The sensor circuit 200 senses incident light 256 and produces an outputsignal representative of the amount of incident light sensed. Thisoutput signal 211 is sent to selector circuit 316. Depending upon thesensor circuit output, the selector circuit delivers a selection signal107 to the delivery circuit 315. If the sensor circuit 200 produces anoutput representative of the presence of high intensity light, such asthe light produced by a welding arc, the selector circuit will send aselection signal 107 to the delivery circuit indicating that a “darkstate” drive signal should be delivered to the shutter assembly 400. Ifthe sensor circuit 200 produces an output representative of the presenceof low intensity light (i.e., no welding arc is present), the selectorcircuit will send a selection signal 107 to the delivery circuitindicating that a “clear state” drive signal should be delivered to theshutter assembly 400. The delivery circuit 315 uses the selection signal107 to determine the voltage level for the drive signals. If theselection signal 315 indicates that a “dark state” drive signal isneeded, the delivery circuit will assemble a drive signal having afrequency set by the frequency signal 102 and voltage levelstransitioning between the voltage signal 104 and the voltage of thepower signal 253. If the selection signal 315 indicates that a “clearstate” drive signal is needed, the delivery circuit will assemble adrive signal having a constant voltage level set by the voltage of thepower signal 253. The delivery circuit 315 delivers this drive signal tothe shutter assembly 400 via drive signal lines 110 and 112. If thedrive signal is a “dark state” drive signal, the shutter assembly 400will be driven to a dark state. If the drive signal is a “clear state”drive signal, the shutter assembly 400 will be driven to a clear state.

Referring to FIG. 2, a detailed schematic of the circuit depicted in theblock diagram of FIG. 1 is shown. Preferably, power supply 250 includesa rechargeable 3 V supply and a solar cell 257. However, a single powersource, either a battery or a solar cell, can be used withoutdramatically altering the operation of the circuit. It is alsopreferable that the solar cell serve as the circuit's primary powersource, with the 3 V supply functioning to provide additional power tovarious circuit components when the solar cell voltage falls below thebattery voltage. The solar cell power supply supplemented by the 3 Vrechargeable supply is seen on line 100 (the signal on this line will bereferred to as the 3 V signal). The solar cell power supply that is notsupplemented by the 3 V rechargeable supply is seen on line 150. Becauseof the improved power efficiency of the present invention, the circuitcan utilize unused energy to recharge the 3 V supply.

To implement these preferences, the 3 V supply is coupled to diode D5 asshown, with the output of D5 fed back to its input. Also, diode D6 iscoupled between the output of D5 and V_(SOL+) as shown. Capacitor C11 iscoupled between V_(SOL+) and ground as shown. C11, preferably 0.1 μF,serves as a filter for V_(SOL+). Capacitors C10 and C13, coupled betweenthe 3 V supply and ground, function to filter and smoothen the 3 Vsupply on line 100. Preferably, C10 and C13 are each 3.3 μF. When light256 reaches the solar cell, the voltage V_(SOL+) will increase until thesolar cell reaches approximately 3.3 V (depending on the amount ofincident light). The solar cell also functions to recharge the 3 Vsupply.

The power supply 250 delivers a 3 V signal along line 100 to the V_(CC)pin of the 4060 chip 310, to the V_(CC) pin of the 4053 chip 320, to theX1 pin of the 320 (through R16), to the Y1 pin of 320, to the Z0 pin of320 and to the collector of transistor Q3 (through resistor R15). Thepower supply delivers V_(SOL+) along line 150 to the collector of thephototransistor S1, to the supply for amplifier 210, and to the base oftransistor Q1 (through resistor R5).

When light incident on the solar cell increases, thereby causing anincrease in voltage on line 150, the voltage at the base of controltransistor Q1 will increase. Once the voltage at the base of Q1 reachesapproximately 0.6 V, Q1 will turn “on” to activate the signal generator325. By only activating the signal generator when there is lightincident on the solar cell, the power drain on the circuit is reducedbecause the signal generator will only be active when the welding helmetis likely to be in use. When the welding helmet is not in use, it istypically left in either a dark room or with its solar cell face down,which in either case would prevent the solar cell from triggering Q1.With Q1 “off,” the signal generator will not drain the power supply.When the welding helmet is in use, it will be exposed to outdoor light,indoor light, or weld light. In these situations, the solar cell willtrigger Q1 to activate the signal generator.

The voltage at the base of Q1 is set by the resistor divider circuitformed by the junction 101 of R5 and R7 as shown. Preferably, R5 is 2 MΩand R7 is 1 MΩ. The emitter of Q1 is grounded. The collector of Q1 isconnected to the 3 V supply through R12. The collector of Q1 is alsoconnected to the RESET pin of 310 via line 105. The signal on line 105serves to activate the signal generator 325. Once Q1 is turned “on,” thevoltage at the collector of Q1 will change from 3 V to substantially 0 Vas a path to ground is created. This transition causes the signal online 105 to go from high to low, which removes the reset signal from310. With the reset signal low, 310 begins toggling C_(IN) and C_(OUT).The two C_(OUT) pins and the C_(IN) pin of 310 are connected to a chargepump 324 as shown. Charge pump 324 comprises capacitors C2, C3, C4, C5,C6, and C7 and diodes D1, D2, D3, and D4 as shown. The charge pump 324functions to generate the signal used to set the voltage level of the“dark state” drive signal. This voltage level is sent to the deliverycircuit 315 via line 104. Preferably, this voltage level is set to beapproximately −15 V using charge pump capacitors C2 through C7 of 0.1 μFapiece. The −15 V signal on line 104 is stored in capacitor C8, which ispreferably 6.8 μF. RC circuit 327 sets the frequency of the charge pump.Preferably, the RC circuit is designed to produce a frequency ofapproximately 550 Hz using an R1 of 2 MΩ, an R2 of 1 MΩ, and a C1 of 680μF. This frequency can be passed through a binary counter in chip 310 todivide the frequency to about 0.5 Hz. The 0.5 Hz signal exits 310through line 102 at pin Q10 as shown. Line 102 delivers this frequencysignal to pins B and C of the 4053 chip 320. This frequency serves asthe frequency for the “dark state” drive signal.

The output of the sensor circuit on line 211 is coupled to the base ofcontrol transistor Q3. The collector of Q3 is connected to a 3 V powersupply through R15. The collector of Q13 is also coupled to the groundthrough capacitor C14. The collector of Q3 is also coupled to pin A ofthe 4053 chip 320 via line 107.

Pins B and C of the chip 320 receive the frequency signal 102 fromsignal generator 325. Thus, pins B and C toggle at the frequency offrequency signal 102, which is preferably 0.5 Hz. Pin A controls theselection of pins X0 and X1. When pin A is “high,” pin X1 is selected.When pin A is “low,” pin X0 is selected. Pin B controls the selection ofpins Y0 and Y1. When pin B is “high,” pin Y1 is selected. When pin B is“low,” pin Y0 is selected. Pin C controls the selection of pins Z0 andZ1. When pin C is “high,” pin Z1 is selected. When pin C is “low,” pinZ0 is selected. The selection of a particular pin means that the signalon the selected pin will be passed on to the output associated with thepair. For example, when pin A is “high,” pin X1 is selected and thesignal at pin X1 is passed through to output pin X. When pin B is“low,”pin Y0 is selected and the signal at pin Y0 is passed through to outputpin Y.

Pin X0 is connected to the B15 V voltage signal supplied on line 104 bysignal generator 325. Pin X1 is connected to the 3 V power supply viaR16. Thus, the status of the signal at pin A determines whether a B15 Vsignal or a 3 V signal is passed through to output pin X. Pin Y0 isconnected to output pin X via line 106. Thus, whatever signal is passedto X will be received at Y0. Pin Y1 is connected to the 3 V supply. PinZ0 is also connected to the 3 V supply. Pin Z1, like pin Y0, isconnected to output pin X via line 106. Output pins Y and Z areconnected to the shutter assembly 400.

The signal on line 211 at the output of the sensor circuit 200 controlswhether Q3 is turned “off” or “on.” Q3 needs a signal on line 211 ofabout 0.6 V to turn “on.” The sensor circuit 200 is configured toproduce an output of at least 0.6 V when a welding arc is present. If nowelding arc is present, Q3 will not receive a sufficient voltage on line211 to turn “on.”

When in the “off” state, the voltage on line 107 will be “high,” thatis, substantially equal to the 3V supply. While pin A is “high,” thesignal at pin X1 is passed through to output pin X. Since pin X1 issubstantially 3 V, this signal will be passed to input pins Y0 and Z1.Thus, when pin A is “high” (which corresponds to no welding arc beingpresent), pins Y0, Y1, Z0, and Z1 will all receive a substantially 3 Vsignal. Thus, as pins B and C alternate from “high” to “low” at 0.5 Hz(the frequency of signal 102) and pins Y0 and Z0, and pins Y1 and Z1 arealternately passed through to output pins Y and Z, the resultant signalon lines 110 and 112 will be a substantially steady 3 V signal. Thissteady 3 V signal on lines 110 and 112 corresponds to a “clear state”drive signal, that is, the drive signal which will transition theshutter assembly to a clear state.

When the output of the sensor circuit 200 is sufficient to turn “on” Q3(indicating the presence of a welding arc), the signal on line 107 willquickly go from 3 V to 0 V as a path to ground is created through Q3.Thus, pin A will go “low.” When pin A is “low,” the signal at pin X0 ispassed through to output pin X. Since pin X0 is B15 V, this B15 V signalwill be received at pins Y0 and Z1. As pins B and C alternate from“high” to “low” at 0.5 Hz (the frequency of signal 102), the value of Ywill be 3 V when the value of Z is B15 V and vice versa. The signal online 110 will alternate between 3 V and B15 V at 0.5 Hz. The signal online 112 will alternate between 3 V and B15 V at 0.5 Hz out of phasewith the signal on line 110. Thus, the resultant signal delivered to theshutter assembly 400 will be an 18 V square wave having a 0.5 Hzfrequency. This 18 V, 0.5 Hz, square wave corresponds to a “dark state”drive signal, that is, the drive signal which will transition theshutter assembly to a dark state.

When the welding arc ceases, the voltage on line 211 will beinsufficient to maintain Q3 in an “on” state. Once Q3 turns “off,” thesignal on line 107 will return to a “high” state. However, thistransition will not be instantaneous due to the RC circuit formed by R15and C14. The transition of 107 from “low” to “high” will be delayed asC14 charges. By selecting the RC time constant for R15 and C14, thedelay can be set to accommodate brief “off” periods in the “on/off”pulsating light of various welding conditions. Before C14 recharges, thelight pulse of the weld arc will pass through the sensor circuit 200 andreactivate Q3 to cause a quick transition on line 107 back to “low.”Preferably R15 is 2 MΩ and C14 is 0.1 μF. The “low-to-high” transitionon line 107 will be about 0.25 seconds in a circuit with thoseparameters.

Sensor circuit 200 includes a phototransistor S1 coupled to a feedbackcircuit 249. Additionally, resistor R13 is coupled between the base andemitter of the phototransistor. The output of phototransistor S1 is sentto line 208. A load resistor R6 is connected between line 208 andground. Additionally, a capacitor C9 couples line 208 to line 209.Resistor R4 is connected between line 209 and ground. Line 209 is alsoconnected to the noninverting input of amplifier 210. Amplifier 210 ispreferably configured as closed loop noninverting amplifier wherein theR9 and R3 feedback loop is connected to the inverting input of amplifier210 as shown. The output of amplifier 210 on line 211 serves as thesensor circuit output. Line 211 is connected to the input of selectorcircuit 316.

The solar cell 257 powers phototransistor S1 and amplifier 210 via line150. Thus, if the solar cell is left unexposed to incident light,phototransistor S1 and amplifier 210 will not receive power, thuspreventing the phototransistor and amplifier from draining the powersupply when the welding helmet is not in use (when not in use, thewelding helmet is typically not exposed to light).

The feedback circuit 249 for the phototransistor S1 comprises a resistorcapacitor circuit 248 connected between the emitter of thephototransistor and ground, and a feedback transistor Q2 having a basecoupled to line 205 of the resistor capacitor circuit 248, a collectorcoupled to the base of the phototransistor, and an emitter coupled tothe ground via resistor R10.

Phototransistor S1 serves as the weld sensor. It receives an input ofincident light 256 and produces an output on line 208 representative ofthe intensity of the incident light. The phototransistor S1 used in thepresent invention is preferably a planar phototransistor configured fora surface mount. The planar phototransistor is smaller than conventionalmetal can phototransistors, thus allowing a reduction in size of theunit in which the sensor circuit is implemented. While the metal canphototransistors used in the sensor circuits of the prior art had athickness of about ½ inch, the planar phototransistors with a surfacemount used in the present invention have a thickness of only about ¼inch. This reduction is thickness allows the sensor circuit to beimplemented into a smaller and sleeker unit. Further, the surface mountconfiguration of the phototransistor S1 allows the phototransistor to beeasily affixed to a circuit board. The inventor herein has found thatthe TEMT4700 silicon npn phototransistor manufactured byVishay-Telefunken is an excellent phototransistor for the presentinvention as it has a smaller size than conventional metal canphototransistors and allows the sensor circuit to maintain a constantsignal level without excessive loading or the drawing of excessivecurrent.

The resistor capacitor circuit 248 and the feedback transistor Q2 in thephototransistor feedback circuit 249 function to adjust the sensitivityof the phototransistor S1. The resistors R8 and R11 and capacitor C12are chosen to be of a size to provide a relatively large time constant,and therefore a relatively slow response to changes in voltage on line208. The delay exists because of the time it takes for the voltage online 205 to charge to an amount sufficiently large to activate Q2.Exemplary values for R8 and R11 are 1 MΩ and 2 MΩ respectively. Anexemplary value for C12 is 0.1 μF. A detailed description of theoperation of the resistor capacitor circuit 248 and feedback transistorQ2 can be found in prior U.S. Pat. Nos. 5,248,880 and 5,252,817, thedisclosures of which have been incorporated by reference.

R13 functions to attenuate phototransistor output in response to lowintensity incident light by essentially shutting down thephototransistor when low intensity light is present. R13 further aidsthe response of the phototransistor by enabling the phototransistor tosharply increase its output when high intensity light is detected. R10,connected between the emitter of Q2 and ground further improves thesensor circuit by attenuating phototransistor output in response to lowintensity light signals. Load resistor R6 is coupled betweenphototransistor output 208 and ground helps to further attenuatephototransistor output when low intensity light is incident upon thephototransistor. An exemplary value for R10 is 20 kΩ. An exemplary valuefor R6 is 1 MΩ.

Referring to FIGS. 3 a, 3 b, and 3 c, the operation of the sensorcircuit 200 will be described. First, the phototransistor hasoperational characteristics similar to a photodiode whose output is fedinto the base of a conventional npn transistor. The equivalent circuitfor a phototransistor is depicted in FIGS. 3 a, 3 b, and 3 c. Photodiode221 is connected between the base and collector of npn transistor 222.Incident light will produce a photocurrent, I_(PHOTO), from thephotodiode 221. I_(PHOTO) serves to feed the base of the transistor 222.However, in the sensor circuit of the present invention, resistor R13 isalso coupled between the base and emitter of the phototransistor. Thus,in the equivalent circuit model, R13 is connected between the base andemitter of transistor 222 as shown.

When light 256 first reaches the phototransistor, the phototransistor S1is in the “off” state. Additionally, feedback transistor Q2 is in the“off” state. FIG. 3 a depicts the equivalent circuit model for thesensor circuit 200 in this mode of operation. In the equivalent circuitmodel, the photocurrent, I_(PHOTO), sees an essentially open circuit inthe path to the base of transistor 222 because transistor 222 is “off.”Thus, I_(PHOTO) passes through R13 as shown in FIG. 3 a. The voltagedrop across R13 caused by I_(PHOTO) will be equal to the base-emittervoltage drop across transistor 222 because R13 is coupled between thebase and emitter of 222. To turn “on” the transistor 222, the voltagedrop across the base and emitter of transistor 222 needs to be about0.47 V to 0.53 V. By selecting a value of R13 that will keep the voltagedrop across R13 below 0.47 V to 0.53 V in response to a photocurrentthat corresponds with low intensity incident light, R13 can attenuatethe phototransistor's output in response to low intensity incidentlight. An exemplary value for R13 is 10 MΩ. Because the phototransistoris not turned “on,” the photocurrent is kept away from the base,preventing amplification of the photocurrent (the base of the transistor222 drives the gain of the phototransistor S1). When transistor 222 isturned “on,” photocurrent will feed the base of transistor 222, and theoutput of the phototransistor will be amplified accordingly. Once online 208, the photocurrent will be further diverted to ground throughR6, through the resistor capacitor circuit 248, and through R4 (via C9).The current passing through the resistor capacitor circuit 248 willbegin the charging of capacitor C12 at line 205.

As more light reaches the phototransistor, I_(PHOTO) will increase. WhenI_(PHOTO) is sufficiently large to create a voltage drop across R13 ofabout 0.47 V to 0.53 V, the transistor 222 will turn “on.” Also, ifintense incident light, such as light from a welding arc, reaches thephototransistor, a large photocurrent will be produced. The largephotocurrent passing through R13 will quickly create a voltage dropacross R13 that is sufficient to turn “on” transistor 222, thusachieving a sharp increase in phototransistor output in response to highintensity light. While in the preferred embodiment R13 is a resistor, itis conceivable that any nonreactive element providing a quick voltagedrop in response to a current may be used in the invention.

When transistor 222 first activates, the feedback transistor Q2 willstill be in the “off” state while it waits for the voltage on line 205to charge through capacitor C12. FIG. 3 b depicts the equivalent circuitmodel for the sensor circuit in this mode of operation. Part ofI_(PHOTO) will be fed into the base of transistor 222 and part ofI_(PHOTO) will be diverted through R13. The current fed into the base of222 will drive the gain for the phototransistor. The output of thephototransistor on line 208 will be the sum of the emitter current oftransistor 222 and the current diverted through R13. Once on line 208,the current will be further diverted to ground through R6, through theresistor capacitor circuit 248, and through R4 (via C9).

As previously explained, the current passing through the resistorcapacitor circuit 248 will cause the capacitor C12 to charge. As C12charges, the voltage on line 205 will begin to increase toward 0.6 V.Once the voltage on line 205 reaches about 0.6 V, the feedbacktransistor Q2 is turned “on.” Once Q2 is activated, it drains some ofthe photocurrent away from the base of the transistor 222 as shown inFIG. 3 c. By diverting photocurrent from the base of thephototransistor, the feedback transistor Q2 decreases the gain providedby the phototransistor, thereby causing a drop in the phototransistoroutput despite an incident light level that remains essentiallyconstant. This biasing operation allows the phototransistor to maintaina constant signal level for a steady light intensity.

The signal on line 208 if fed into an amplifier 210. The signal is firstpassed through a capacitor C9 to block the DC component of the detectedsignal. Line 209 contains the DC blocked detected signal. The current online 209 is diverted to ground via resistor R4.

The sensor circuit operates in the presence of both AC welds and DCwelds. In an AC weld (also known as a MIG weld), the welding light ispulsating. Thus, the phototransistor will detect a pulsating lightsignal. The frequency of the pulsations is often 120 Hz. In a DC weld(also known as a TIG weld), the welding light is substantiallycontinuous, with the exception of a small AC component. When an AC weldis present, the phototransistor will produce a pulsating output on line208. The variations in the voltage signal due to the pulses will bepassed through the capacitor to line 209 and fed into the amplifier. Theamplifier will then provide gain for the signal on line 209 which issufficient to trigger the delivery of a “dark state” drive signal to theshutter assembly 400. The slow charge time of capacitor C14 in selectorcircuit 316 will prevent the transition from a dark state to a clearstate during brief interruptions in the AC weld pulses. Before C14recharges, the next AC pulse will cause the capacitor to dischargebefore a “clear state” drive signal is triggered.

When a DC weld is present, the phototransistor will quickly produce anoutput on line 208 catching the rising edge of the DC weld. This suddenrise in voltage on line 208 will be passed through to the amplifier 210causing a signal on line 211 sufficient to trigger the delivery of a“dark state” drive signal to the shutter assembly 400. Thereafter,capacitor C9 will block the DC component of the DC weld, allowing onlythe AC variations in the DC weld to pass through to the amplifier. Theamplifier 210 must have a gain sufficient to keep the shutter assemblyin the dark state when the AC variations in the DC weld reach theamplifier.

The amplifier 210 is a closed loop, noninverting amplifier as describedabove. The output of the amplifier is fed into a selector circuit 316.The selector circuit 316 uses a phototransistor to send a selectionsignal to the delivery circuit 315 via line 107. As previouslyexplained, for the selector circuit 316 to send a signal indicating thata “dark state” drive signal should be delivered to the shutter assembly,a 0.6 V signal needs to be applied to the base of control transistor Q3on line 211. Thus, it can be seen that amplifier 210 must produce asignal of about 0.6 V on line 211 when the phototransistor produces asignal on line 208 indicative of the presence of a welding arc. The gainof amplifier 210 must therefore be set such that it will sufficientlyamplify its input voltage to produce an output voltage of about 0.6 Vwhen the input signal on line 209 indicates the presence of a weldingarc. The gain of the amplifier 210 is set by resistors R9 and R3 in thefeedback loop. The gain of the amplifier having this configuration is:Gain=(R 9/R 3)+1

The inventor herein has noted that a gain of about 3.67 will besufficient for the amplifier to trigger the “dark state” drive signalwhen a welding arc is lit. Exemplary values for R9 and R3 would be 1 MΩand 374 kΩ respectively.

Referring to FIG. 4, the output of the phototransistor will be describedin relation to the amount of incident light. The curve of FIG. 4 depictsthe output of the phototransistor on line 208 (on the vertical axis) asa function of the intensity of incident light (on the horizontal axis).The curve has a relatively steep portion 241 for lower intensityincident light and a less steep portion 242 for higher intensityincident light. The operation of the phototransistor in these portionsof the curve are discussed in detail in prior U.S. Pat. Nos. 5,248,880and 5,252,817, the disclosures of which have been incorporated byreference. Of note for the present invention is curve portion 243 whichrepresents an extremely low voltage response from the phototransistorwhen the incident light has low intensity. This gap in the voltageresponse of the phototransistor is due to the effect of R13 whereby itprevents the activation of the phototransistor in the presence of lowintensity light. However, the invention still provides a sharp increasein phototransistor output when the light intensity increases as can beseen by the steep slope of curve portion 241.

The invention has been disclosed herein in the context of the inventor'spreferred embodiment. However, changes and modifications thereto aswould be apparent to one of ordinary skill in the art are intended to beincluded by the inventor and the invention should be limited only by thescope of the claims appended hereto, and their equivalents.

1. An auto darkening eye protection device comprising: a shutterassembly, the shutter assembly being adjustable to a plurality of shadelevels; a light sensing circuit for sensing light from a welding arc andproviding an output indicative of the shade level at which the shutterassembly should be operated, the light sensing circuit comprising: aphototransistor operative to receive the light from the welding arc andproduce a phototransistor output representative thereof, thephototransistor output from the phototransistor being used to form theoutput of the light sensing circuit, the phototransistor beingconfigured for surface mount and having an external base connectionconnected to the base of the phototransistor; a control circuitconfigured to receive the output from the light sensing circuit andprovide a drive signal to the shutter assembly responsive to saidoutput, the drive signal being operative to drive the shutter assemblyto one of said plurality of shade levels; and a power source coupled tothe light sensing circuit and control circuit for powering same.
 2. Theauto darkening eye protection device of claim 1 wherein the lightsensing circuit further comprises an amplifier coupled to thephototransistor and the control circuit for amplifying thephototransistor output.
 3. The auto darkening eye protection device ofclaim 2 wherein the amplifier comprises a closed loop non-invertingamplifier.
 4. The auto darkening eye protection device of claim 2wherein the light sensing circuit further comprises a capacitor incircuit between the phototransistor and the amplifier for blocking DC inthe phototransistor output.
 5. The auto darkening eye protection deviceof claim 2 wherein the amplifier provides a gain to the output of thephototransistor output sufficient to trigger the control circuit todrive the shutter assembly to a dark shade level when thephototransistor receives a light input having an intensity indicative ofa welding arc being present.
 6. The auto darkening eye protection deviceof claim 1, further comprising a resistor coupled between the base andemitter of the phototransistor for triggering the phototransistor. 7.The auto darkening eye protection device of claim 1 wherein the lightsensing circuit further comprises a feedback circuit connected to thephototransistor for biasing the phototransistor.
 8. The auto darkeningeye protection device of claim 1 the power source includes a solar cell,and wherein the control circuit comprises a signal generator forgenerating a drive signal, and an electronic switch for activating thesignal generator, the electronic switch having an input connected to thesolar cell and an output connected to the signal generator, the controlcircuit being configured to activate the signal generator when the solarcell powers the electronic switch.
 9. The auto darkening eye protectiondevice of claim 1 wherein the control circuit further comprises: atransistor for controlling the transition of the shutter assembly to oneof a plurality of shade levels; and an RC circuit coupled to the secondtransistor for delaying a transition in the shutter assembly from a darkshade level to a clear shade level.
 10. The auto darkening eyeprotection device of claim 1 wherein the power supply comprises a solarcell and a secondary power source, the power supply being configuredsuch that the solar cell recharges the secondary power source.
 11. Theauto darkening eye protection device of claim 10 wherein the secondarypower source will supplement the power supplied by the solar cell toprovide power to a circuit component, and wherein the solar cellprovides power solely to another circuit component with such power notbeing supplemented by the secondary power source.
 12. The auto darkeningeye protection device of claim 11 wherein the circuit component providedpower solely by the solar cell includes the light sensing circuit. 13.The auto darkening eye protection device of claim 11 wherein the circuitcomponent provided power solely by the solar cell includes thephototransistor.
 14. A welding helmet comprising: a helmet; and an autodarkening eye protection device comprising: a shutter assembly mountedto the helmet wherein a user of the helmet can see through the shutterassembly, the shutter assembly being adjustable to a plurality of shadelevels; a light sensing circuit for sensing light from a welding arc andproviding an output indicative of the shade level at which the shutterassembly should be operated, the light sensing circuit comprising: aphototransistor operative to receive the light from the welding arc andproduce a phototransistor output representative thereof, thephototransistor output from the phototransistor being used to form theoutput of the light sensing circuit, the phototransistor beingconfigured for surface mount and having an external base connectionconnected to the base of the phototransistor; a control circuitconfigured to receive the output from the light sensing circuit andprovide a drive signal to the shutter assembly responsive to saidoutput, the drive signal being operative to drive the shutter assemblyto one of said plurality of shade levels; and a power source coupled tothe light sensing circuit and control circuit for powering same.
 15. Thewelding helmet of claim 14 further comprising an amplifier connectedbetween the light sensing circuit and the control circuit for amplifyingthe phototransistor output.
 16. The welding helmet of claim 15 whereinthe amplifier comprises a closed loop non-inverting amplifier.
 17. Thewelding helmet of claim 15 wherein the amplifier provides a gain to theoutput of the light sensing circuit sufficient to trigger the controlcircuit to drive the shutter assembly to a dark shade level when thephototransistor receives a light input having an intensity indicative ofa welding arc being present.
 18. The welding helmet of claim 15 furthercomprising a capacitor in circuit between the phototransistor and theamplifier for blocking DC in the phototransistor output.
 19. The weldinghelmet of claim 14 further comprising a nonreactive element coupled tothe base of the phototransistor for controlling the response of thephototransistor to incident light having a wide range of intensity. 20.The welding helmet of claim 14 further comprising a feedback circuitconnected to the phototransistor for biasing it and thereby reducing itsresponsiveness to ambient light.
 21. The welding helmet of claim 14wherein the power supply comprises solar cell and a secondary powersource, the power supply being configured such that the solar cellrecharges the secondary power source.
 22. The welding helmet of claim 21wherein the secondary power source will supplement the power supplied bythe solar cell to provide power to a circuit component, and wherein thesolar cell provides power solely to another circuit component with suchpower not being supplemented by the secondary power source.
 23. Thewelding helmet of claim 22 wherein the circuit component provided powersolely by the solar cell is the light sensing circuit.
 24. The weldinghelmet of claim 22 wherein the circuit component provided power solelyby the solar cell is the phototransistor.
 25. The welding helmet ofclaim 14 wherein the power supply includes a solar cell and wherein thecontrol circuit comprises a signal generator for generating a drivesignal, and a electronic switch for activating the signal generator, theelectronic switch having an input connected to the solar cell and anoutput connected to the signal generator, the control circuit beingconfigured to activate the signal generator when the solar cell powersthe electronic switch.
 26. The welding helmet of claim 14 wherein thecontrol circuit comprises a second transistor for controlling transitionof the shutter assembly to one of a plurality of shade levels, and an RCcircuit coupled to the second transistor for delaying the transition ofthe shutter assembly from a dark shade level to a clear shade level.